U.S. patent number RE37,309 [Application Number 09/112,380] was granted by the patent office on 2001-08-07 for scanning exposure apparatus.
This patent grant is currently assigned to Nikon Corporation. Invention is credited to Masato Hamatani, Toshiharu Nakashima, Ken Ozawa.
United States Patent |
RE37,309 |
Nakashima , et al. |
August 7, 2001 |
Scanning exposure apparatus
Abstract
A projection exposure apparatus for transferring a pattern
formed on a mask onto a photosensitive substrate by a scanning
exposure method, includes a light source for generating a light
beam having a predetermined spatial coherence, an illumination
optical system for receiving the light beam from the light source
and illuminating a local area on the mask with the light beam, and
a device for synchronously moving the mask and the photosensitive
substrate so as to transfer the pattern on the mask onto the
photosensitive substrate. A direction, corresponding to a higher
spatial coherence of the light beam, is made to coincide with the
direction of relative scanning an illumination area and the mask in
the illumination area.
Inventors: |
Nakashima; Toshiharu (Kawasaki,
JP), Hamatani; Masato (Kohnosu, JP), Ozawa;
Ken (Tokyo, JP) |
Assignee: |
Nikon Corporation (Tokyo,
JP)
|
Family
ID: |
26473363 |
Appl.
No.: |
09/112,380 |
Filed: |
July 9, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
255927 |
Jun 7, 1994 |
05534970 |
Jul 9, 1996 |
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Foreign Application Priority Data
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Jun 11, 1993 [JP] |
|
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5-141016 |
Nov 19, 1993 [JP] |
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5-290478 |
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Current U.S.
Class: |
355/53; 353/122;
355/67; 359/589; 359/622; 355/71 |
Current CPC
Class: |
G03F
7/70075 (20130101); G03F 7/70583 (20130101); G03F
7/70358 (20130101); G03F 7/70575 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G02B 027/00 () |
Field of
Search: |
;355/53,67,71
;359/589,622,619,370 ;353/122 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Mathews; Alan A.
Attorney, Agent or Firm: Miles & Stockbridge P.C.
Claims
What is claimed is:
1. A scanning exposure apparatus comprising:
a light source for emitting a spatially coherent light beam;
an illumination optical system for radiating the light beam from
said light source onto a mask so as to form an illumination area on
a local area of the mask, said light beam having higher spatial
coherence in one direction than in another direction of a
cross-section of the beam in the illumination area; and
a device for synchronously moving the mask and a photosensitive
substrate so as to transfer a pattern formed on the mask onto the
photosensitive substrate,
wherein a direction of relative scanning of the illumination area
and the mask upon movement of the mask substantially coincides with
the direction with higher spatial coherence of the light beam.
2. An apparatus according to claim 1, wherein said light source
comprises a pulse oscillation type laser light source for emitting
a light beam in a deep ultraviolet range.
3. An apparatus according to claim 1, further comprising:
a projection optical system for projecting an image of the pattern
in the illumination area onto photosensitive substrate,
wherein said moving device comprises a mask stage which holds the
mask to be substantially perpendicular to an optical axis of said
projection optical system, and moves substantially along the
direction with the higher spatial coherence of the light beam, and
a substrate stage which holds the photosensitive substrate and
moves in a direction substantially perpendicular to the optical
axis of said projection optical system.
4. An apparatus according to claim 1, further comprising:
an optical member for receiving the light beam from said light
source, and shifting an interference pattern formed in the
illumination area in a direction substantially perpendicular to the
direction of relative scanning.
5. An apparatus according to claim 4, wherein said illumination
optical system comprises an optical integrator for receiving the
light beam, and forming a plurality of light source images, and
said optical member changes an incident angle of the light beam or
said optical integrator.
6. An apparatus according to claim 1, further comprising:
a device for shifting an interference pattern in the illumination
area along the direction of relative scanning in accordance with a
moving speed of the mask and an intensity distribution, in the
direction of relative scanning, of the interference pattern.
7. An apparatus according to claim 6, further comprising:
a device for detecting spatial coherence of the light beam by
receiving at least a portion of the light beam from said light
source; and
a controller for controlling an operation of said shifting device
in accordance with the detected spatial coherence.
8. A scanning exposure apparatus comprising:
a light source for emitting a spatially coherent light beam;
an illumination optical system for radiating the light beam from
said light source onto a mask so as to form an illumination area on
a local area of the mask;
a device for synchronously moving the mask and a photosensitive
substrate so as to transfer a pattern formed on the mask onto the
photosensitive substrate; and
a device for shifting an interference pattern in the illumination
area in accordance with a moving speed of the mask and an intensity
distribution of the interference pattern while the mask and the
photosensitive substrate are being synchronously moved.
9. An apparatus according to claim 8, wherein the light beam has
higher spatial coherence in one direction than in another direction
of a cross-section of the beam in the illumination area, said
moving device comprises a mask stage which holds the mask and moves
substantially along the direction with higher spatial coherence of
the light beam, and a substrate stage which holds the
photosensitive substrate and is movable in synchronism with
movement of said mask stage.
10. An apparatus according to claim 8, further comprising:
a device for detecting spatial coherence of the light beam by
receiving at least a portion of the light beam from said light
source; and
a controller for controlling an operation of said shifting device
in accordance with the detected spatial coherence.
11. A scanning exposure apparatus comprising:
a light source for emitting spatially coherent pulsed light;
an illumination optical system for radiating the light from said
light source onto a mask so as to form an illumination area locally
on the mask;
a device for synchronously moving the mask and a photosensitive
substrate so as to transfer a pattern formed on the mask onto the
photosensitive substrate; and
a device for shifting an interference pattern formed in the
illumination area in a moving direction of the mask,
wherein an accumulated light amount on the mask or the
photosensitive substrate, upon radiation of a plurality of light
pulses from said light source onto said mask, is uniformed by
operation of said moving device and said shifting device.
12. An apparatus according to claim 11, wherein said shifting
device comprises an optical member for receiving light pulses from
said light source and varying a propagation direction of the light
pulses, and shifts the interference pattern in the illumination
area in the moving direction of the mask and a direction
substantially perpendicular to the moving direction.
13. A scanning exposure apparatus comprising:
a light source;
an illumination optical system for illuminating a mask with a light
beam from said light source, said light beam having higher spatial
coherence in one direction than in another direction of a
cross-section of the beam in an illumination area on the mask;
a projection optical system for projecting an image of a pattern
formed on the mask onto a photosensitive substrate;
a mask stage which holds the mask to be substantially perpendicular
to an optical axis of said projection optical system, and which
moves substantially along the direction with higher spatial
coherence of the light beam;
a substrate stage which holds the photosensitive substrate and
moves in a direction substantially perpendicular to the optical
axis of said projection optical system; and
a device for synchronously driving said mask stage and said
substrate stage so as to transfer the pattern on the mask onto the
photosensitive substrate.
14. An apparatus according to claim 13, further comprising:
a device for shifting an interference pattern formed on the mask in
a moving direction of said mask stage or in a direction crossing
the moving direction, in association with the movement of said mask
stage.
15. A scanning exposure apparatus comprising:
a projection optical system for projecting an image of a pattern
formed on a mask onto a photosensitive substrate;
a device for synchronously moving the mask and the photosensitive
substrate along a direction substantially perpendicular to an
optical axis of said projection optical system so as to transfer
the pattern on the mask onto the photosensitive substrate; and
a device for illuminating a local area on the mask with light beam
having higher spatial coherence in one direction than in another
direction of a cross-section of the beam, such that the direction
with higher spatial coherence of the light beam substantially
coincides with a moving direction of the mask.
16. An apparatus according to claim 15, wherein said illuminating
device comprises a pulse laser light source for emitting the light
beam, an optical member for substantially aligning the direction
with higher spatial coherence of the light beam with the moving
direction of the mask, a field stop arranged on a plane
substantially conjugate with a pattern surface of the mask, and a
light transmission system for guiding, to the mask, light which
emerges from said optical member and passes through an aperture of
said field stop.
17. An apparatus according to claim 16, further comprising:
a device for shifting an interference pattern formed on the mask in
the moving direction of the mask or a direction crossing the moving
direction.
18. An apparatus for illuminating a mask having a pattern to be
transferred onto a photosensitive substrate by a scanning exposure
method, comprising:
a light source;
a device for radiating the light beam from said light source onto
the mask via an aperture of a field stop arranged on a plane
substantially conjugate with a pattern surface of the mask, said
light beam having higher spatial coherence in one direction than in
another direction of a cross-section of the beam in an illumination
area on the mask; and
an optical member for substantially aligning the direction with
higher spatial coherence of the light beam with a scanning
direction of the mask. .Iadd.
19. A scanning exposure method comprising:
radiating a spatially coherent light beam onto a mask so as to form
an illumination area; and
synchronously moving the mask and a substrate so as to transfer a
pattern formed on the mask onto the substrate; and
shifting an interference pattern formed in the illumination area in
a moving direction of the mask. .Iaddend..Iadd.
20. A scanning exposure method comprising:
synchronously moving a mask and a substrate so as to transfer a
pattern formed on the mask onto the substrate; and
illuminating the mask with a light beam having higher spatial
coherence in one direction than in another direction of a
cross-section of the beam, such that the direction with higher
spatial coherence of the light beam substantially coincides with a
moving direction of the mask. .Iaddend..Iadd.
21. A scanning exposure method comprising:
moving, during a scanning exposure, a mask and an object relative
to an exposure beam to expose the object with a pattern formed on
the mask; and
directing the exposure beam to an irradiation area during the
scanning exposure, the mask moving relative to the irradiation
area, in a scanning direction during the scanning exposure,
wherein the exposure beam has higher spatial coherence in one
direction than in another direction, and wherein said one direction
substantially coincides with the scanning direction.
.Iaddend..Iadd.
22. A method according to claim 21, further comprising:
shifting an interference pattern formed in the irradiation area to
which the exposure beam is directed. .Iaddend..Iadd.
23. A method according to claim 22, wherein the interference
pattern is shifted in accordance with a relative velocity of said
mask and said exposure beam. .Iaddend..Iadd.
24. A method according to claim 22, wherein the interference
pattern is shifted in accordance with an intensity distribution of
the interference pattern. .Iaddend..Iadd.
25. A method according to claim 22, wherein the interference
pattern is shifted in the scanning direction. .Iaddend..Iadd.
26. A method according to claim 22, wherein the interference
pattern is shifted in a direction transverse to the scanning
direction. .Iaddend..Iadd.
27. A method according to claim 26, wherein said direction
transverse to the scanning direction includes a direction
perpendicular to the scanning direction. .Iaddend..Iadd.
28. A scanning exposure apparatus in which a work-piece is moved in
a scanning direction relative to an exposure beam during a scanning
exposure, comprising:
a projection system, disposed in a path of said exposure beam,
which projects a pattern image onto the work-piece, the exposure
beam having higher spatial coherence in one direction than in
another direction; and
an optical system, disposed in the path of said exposure beam,
which directs said exposure beam to said work-piece such that said
one direction substantially coincides with the scanning direction.
.Iaddend..Iadd.
29. A scanning exposure apparatus according to claim 28, further
comprising:
a shifting member, disposed in the path of said exposure beam,
which shifts an interference pattern formed in an irradiation area
of the exposure beam during the scanning exposure.
.Iaddend..Iadd.
30. An apparatus according to claim 29, wherein said shifting
member shifts the interference pattern in the scanning direction.
.Iaddend..Iadd.
31. An apparatus according to claim 29, wherein said shifting
member shifts the interference pattern in a direction transverse to
the scanning direction. .Iaddend..Iadd.
32. An apparatus according to claim 28, wherein said optical system
includes an optical integrator. .Iaddend..Iadd.
33. A scanning exposure method comprising:
moving, during a scanning exposure, a mask and an object relative
to an exposure beam to expose the object with a pattern formed on
the mask;
directing the exposure beam to an irradiation area during the
scanning exposure, the mask moving relative to the irradiation area
in a scanning direction during the scanning exposure; and
shifting an interference pattern formed in the irradiation area
during the scanning exposure. .Iaddend..Iadd.
34. A method according to claim 33, wherein the interference
pattern is shifted in the scanning direction. .Iaddend..Iadd.
35. A method according to claim 34, wherein the interference
pattern is shifted in accordance with a relative velocity of said
mask and said exposure beam. .Iaddend..Iadd.
36. A method according to claim 34, wherein said interference
pattern is shifted in accordance with an intensity distribution of
the interference pattern. .Iaddend..Iadd.
37. A scanning exposure method according to claim 33, wherein the
following condition is satisfied:
where,
V: a relative velocity of said mask and said exposure beam;
E: an optimum exposure dose of said mask;
Ep: an average energy of said exposure beam;
D: a width of said irradiation area in the scanning direction;
f: an oscillation frequency of said exposure beam.
.Iaddend..Iadd.
38. A method according to claim 33, wherein the interference
pattern is shifted in a direction transverse to the scanning
direction. .Iaddend..Iadd.
39. A method according to claim 38, wherein said direction
transverse to the scanning direction includes a direction
perpendicular to the scanning direction. .Iaddend..Iadd.
40. A method according to claim 33, wherein the exposure beam has
higher spatial coherence in one direction than in another
direction, and wherein the interference pattern is shifted in said
one direction. .Iaddend..Iadd.
41. A method according to claim 40, wherein said one direction
substantially coincides with the scanning direction.
.Iaddend..Iadd.
42. A method according to claim 33, wherein the interference
pattern has lower contrast in one direction than in another
direction, and wherein the interference pattern is shifted in said
one direction. .Iaddend..Iadd.
43. A method according to claim 42, wherein said one direction is
transverse to the scanning direction. .Iaddend..Iadd.
44. A method according to claim 33, wherein the interference
pattern has higher contrast in one direction than in another
direction, and wherein the interference pattern is shifted in said
one direction. .Iaddend..Iadd.
45. A method according to claim 44, wherein said one direction
substantially coincides with the scanning direction.
.Iaddend..Iadd.
46. A scanning exposure method in which a work-piece to be exposed
and an exposure beam are moved relatively during scanning exposure,
comprising:
determining an exposure condition of the work-piece based on
information concerning a divergent angle of said exposure beam;
and
effecting scanning exposure based on the determined exposure
condition. .Iaddend..Iadd.
47. A method according to claim 46, wherein said exposure beam is
emitted from a beam source of a pulse-oscillation type, and said
exposure condition is an oscillation cycle of the exposure beam
during the scanning exposure. .Iaddend..Iadd.
48. A method according to claim 46, wherein said exposure beam is
emitted from a beam source of a pulse-oscillation type, and said
exposure condition is a number of pulses of the exposure beam with
which said work-piece is irradiated. .Iaddend..Iadd.
49. A method according to claim 46, wherein said exposure condition
is a shift condition of an interference pattern formed within an
irradiation area of said exposure beam. .Iaddend..Iadd.
50. A method according to claim 46, wherein said exposure condition
is a relative velocity of said exposure beam and said work-piece.
.Iaddend..Iadd.
51. A scanning exposure method comprising:
moving a mask and an object synchronously to expose the object with
a pattern formed on the mask during a scanning exposure; and
directing a plurality of exposure beams to an irradiation area
during the scanning exposure, the mask moving relative to the
irradiation area in a scanning direction during the scanning
exposure,
wherein the plurality of exposure beams have illuminance
distributions in the scanning direction that are different from
each other. .Iaddend..Iadd.
52. A method according to claim 51, wherein said plurality of beams
differ in their directions of polarization to each other.
.Iaddend..Iadd.
53. A scanning exposure method comprising:
directing an exposure beam to an irradiation area;
relatively moving an object and the irradiation area for scanning
exposure; and
determining whether or not an interference pattern formed with said
irradiation area is to be shifted during said scanning exposure.
.Iaddend..Iadd.
54. A method according to claim 53, wherein whether or not the
interference pattern is to be shifted is determined based on a
relative velocity of said irradiation area and said object.
.Iaddend..Iadd.
55. A method according to claim 53, wherein whether or not the
interference pattern is to be shifted is determined based on an
intensity distribution of the interference pattern.
.Iaddend..Iadd.
56. A method according to claim 55, wherein whether or not the
interference pattern is to be shifted is determined based on a
value D/NP,
wherein
D: a width of said irradiation area in a direction of said relative
moving;
N: a number of pulses with which a point on said object is to be
irradiated;
P: a pitch of said interference pattern. .Iaddend..Iadd.
57. A scanning exposure method comprising:
directing an exposure beam to an irradiation area;
relatively moving an object and the irradiation area in a first
direction for scanning exposure; and
shifting an interference pattern formed within the irradiation area
in a first direction and a second direction which is transverse to
said first direction, during the scanning exposure;
wherein a way of controlling the shift of the interference pattern
in said first direction is different from a way of controlling the
shift of the interference pattern in said second direction.
.Iaddend..Iadd.
58. A method of manufacturing a device using an object,
comprising:
moving, during a scanning exposure, a mask and the object relative
to an exposure beam to expose the object with a pattern formed on
the mask; and
directing the exposure beam to an irradiation area during the
scanning exposure, the mask moving relative to the irradiation area
in a scanning direction during the scanning exposure,
wherein the exposure beam has higher spatial coherence in one
direction than in another direction, and wherein said one direction
substantially coincides with the scanning direction.
.Iaddend..Iadd.
59. A method of manufacturing a device using a work-piece,
comprising:
providing a scanning exposure apparatus in which a work-piece is
moved in a scanning direction relative to an exposure beam during a
scanning exposure, and in which a projection system and an optical
system are disposed in the path of the exposure beam;
using the projection system to project a pattern image onto the
work-piece, the exposure beam having higher spatial coherence in
one direction than in another direction; and
using the optical system to direct the exposure beam to the
work-piece such that said one direction substantially coincides
with the scanning direction. .Iaddend..Iadd.
60. A method of manufacturing a device using an object,
comprising:
moving, during a scanning exposure, a mask and the object relative
to an exposure beam to expose the object with a pattern formed on
the mask;
directing the exposure beam to an irradiation area during the
scanning exposure, the mask moving relative to the irradiation area
in a scanning direction during the scanning exposure; and
shifting an interference pattern formed in the irradiation area
during the scanning exposure. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning exposure apparatus,
used in a photolithography process in the manufacture of, e.g., a
semiconductor element, a liquid crystal display element, a
thin-film magnetic head, or the like, for transferring a pattern on
a mask onto a substrate by synchronously moving the mask (or
reticle) and the substrate and, more particularly, to a scanning
exposure apparatus suited for a case wherein light having a high
spatial coherency (e.g., harmonics of a KrF or ArF excimer laser,
YAG laser, or the like) is used.
2. Related Background Art
In the photolithography process for the manufacture of
semiconductor elements, a reduction projection exposure apparatus
(stepper) adopting a step-and-repeat method for transferring a
pattern on a mask or reticle (to be generally referred to as a
"reticle" hereinafter) onto a semiconductor wafer coated with a
photosensitive material (photoresist) via a projection optical
system is used. In a stepper of this type, in order to improve the
resolution by decreasing the wavelength of exposure light, it has
been proposed to use, as exposure light, laser light in a far (or
deep) ultraviolet range, e.g., harmonics or the like of a KrF or
ArF excimer laser, a YAG laser, or an argon laser. At present, a
stepper using the KrF excimer laser has been put into practical
use, and is operating in manufacturing lines.
Laser light generally has a high spatial coherency (coherence) and
forms a speckle pattern (interference fringes) on a reticle. As a
result evenness of the illuminance on the reticle and wafer is
impaired. In view of this, problem as disclosed in, e.g., U.S. Pat.
No. 4,619,508 and Japanese Laid-Open Patent Application No.
1-259533 (corresponding to U.S. Pat. No. 5,307,207 (Mar. 13,
1989)), a pivot mirror is arranged at the light source side of a
fly-eye lens in an illumination optical system to change the
incident angle of laser light onto the fly-eye lens for every one
to several pulses. With this arrangement, the interference fringes
sequentially move on the reticle during exposure. Therefore, the
evenness of the illuminance on the reticle or wafer, i.e., the
evenness of the exposure amount, can be improved.
Recently, it is required to widen the image field of the projection
optical system and to improve its resolution in correspondence with
an increase in size and a decrease in line width of semiconductor
elements. However, it is very difficult in terms of design and
manufacture to realize both the high resolution and wide field of
the projection optical system. Under the circumstances, a scanning
exposure apparatus as disclosed in, e.g., U.S. Pat. Nos. 4,747,678,
4,924,257, and 5,194,893 has been the subject of much attention. In
such an apparatus, a pattern on a reticle is transferred onto a
wafer by illuminating only a local area on the reticle with light
and synchronously moving the reticle and wafer. The scanning
exposure apparatus can transfer a large-area pattern image onto the
wafer even if the image field of the projection optical system is
small, and can relatively easily improve the resolution of the
projection optical system.
In the scanning exposure apparatus, since the reticle and wafer are
synchronously scanned, the relationship between the moving amount
(pitch) of the reticle and wafer and the pitch (in the scanning
direction) of interference fringes in the illumination area between
pulse emissions changes depending upon the scanning speed of the
stage (i.e., an optimal exposure amount of the wafer). Therefore,
when the scanning exposure apparatus uses light having a high
spatial coherency as exposure light, it is difficult to reduce
exposure amount unevenness caused by interference fringes even when
the above-mentioned pivot mirror is used.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a scanning
exposure apparatus which can minimize exposure amount unevenness on
a photosensitive substrate due to interference fringes, even when
light having a high spatial coherency is used as exposure
light.
A first scanning exposure apparatus according to the present
invention comprises a light source for emitting a light beam having
a predetermined spatial coherence, an illumination optical system
for illuminating a local area on a mask with the light beam, and a
device for synchronously moving the mask and a photosensitive
substrate to transfer a pattern formed on the mask onto the
photosensitive substrate, wherein a direction with higher spatial
coherence of the light beam (i.e. a direction in which spatial
coherence of the light beam is high) substantially coincides with
the scanning direction of the mask with respect to the illumination
area in the illumination area on the mask.
According to the first apparatus of the present invention, the
direction in which the spatial coherence (degree of coherence) of
the light beam is high is measured in advance in a plane
perpendicular to the optical axis of the illumination optical
system for guiding the light beam from the light source to the
mask, and the direction with a higher spatial coherence is made to
coincide with the scanning direction of the mask in the
illumination area. Therefore, as shown in, e.g., FIG. 4, the
illuminance distribution, in the scanning direction (X direction),
in the illumination area varies at a predetermined pitch and at a
relatively large amplitude, as indicated by a curve 40. On the
other hand, the illuminance distribution, in the non-scanning
direction (Y direction) perpendicular to the scanning direction, in
the illumination area is relatively flat, as indicated by a curve
41. In this case, even when the illuminance distribution (curve 40)
largely varies in the scanning direction, since the mask is scanned
along the direction corresponding to a higher spatial coherence,
exposure amount unevenness in the scanning direction on the
photosensitive substrate after scanning exposure is remarkably
reduced. Since the illumination distribution (curve 41) in the
non-scanning direction is originally flat, to begin with exposure
amount unevenness in the non-scanning direction on the
photosensitive substrate is also very small. Therefore, even when
illuminance evenness is impaired by interference fringes in the
illumination area, exposure amount unevenness on the entire surface
of the shot area on the photosensitive substrate can be reduced,
i.e., the evenness of the exposure amount can be improved.
A second scanning exposure apparatus according to the present
invention comprises a light source for emitting pulse light having
a predetermined spatial coherence, an illumination optical system
for receiving the pulse light and forming a local illumination area
on a mask with the pulse light, and a device for synchronously
moving the mask and a photosensitive substrate to transfer a
pattern formed on the mask onto the photosensitive substrate. The
apparatus further comprises an interference fringe moving member
for changing a position of interference fringes in the illumination
area for every one to several pulses in accordance with a relative
scanning speed between the illumination area and the mask and a
pitch, in the relative scanning direction, of the interference
fringes formed in the illumination area. The second apparatus may
also comprise a detector for detecting a spatial coherence of the
pulse light, and a controller for controlling the operation of the
interference fringe moving member in accordance with the detected
spatial coherence.
According to the second apparatus of the present invention, pulse
light is used as exposure light. When the pulse light is far
ultraviolet laser light, KrF excimer laser light having a
wavelength of 248 nm, it is not easy to satisfactorily correct the
chrominance aberration of a projection optical system. The pulse
light source therefore preferably narrows the wavelength band of
the pulse light using a diffraction grating (or etalon), a slit,
and the like. For this reason, in FIG. 1, for example, pulse light
(LB.sub.0) emitted from a light source (1) has a high spatial
coherence and a small beam width in the horizontal direction (H
direction) but has a low spatial coherence and a large beam width
in the vertical direction (V direction). Therefore, the horizontal
direction of the pulse light emitted from the light source is set
in the scanning direction of the illumination area on the mask.
Note that the ratio between the widths, in the horizontal and
vertical directions, of the pulse light from the light source is
generally smaller than the ratio between the widths, in the
scanning and non-scanning directions, of the illumination area. For
this reason, the width, in the horizontal direction, of the pulse
light is increased using two cylindrical lenses (38, 39), as shown
in, e.g., FIG. 3. If the divergent angle of incident pulse light
(LB.sub.0) is represented by .theta..sub.1, the focal length of the
front-side (light source-side) cylindrical lens 38 is represented
by f.sub.1, and the focal length of the rear-side (mask-side)
cylindrical lens 39 is represented by f.sub.2, a divergent angle
.theta..sub.2 of pulse light (LB) emerging from the cylindrical
lens (39) is given by the following equation:
If f.sub.1 <f.sub.2 is set to increase the beam width in the
horizontal direction, the following relation is satisfied, and the
divergent angle .theta..sub.2 of the emerging pulse light (LB)
decreases:
Therefore, when the beam width is increased in the horizontal
direction, the spatial coherence in the scanning direction (SR
direction) of the illumination area further increases, as shown in
FIG. 4. For this reason, interference fringes with a high contrast
are formed in the scanning direction. Since the contrast, in the
non-scanning direction, of the interference fringes is low,
illuminance unevenness in the non-scanning direction becomes
sufficiently small.
The illuminance distribution in the scanning direction of the
illumination area is as indicated by, e.g., a curve 40 in FIG. 5A.
Therefore, when the scanning direction of the mask (and the
photosensitive substrate) is selected in this direction, waves of
various phases are superposed on the photosensitive substrate due
to relative movement between the interference fringes and the mask
by scanning, as shown in FIG. 5B, whereby exposure amount
unevenness caused by the interference fringes can be remarkably
reduced by the accumulation effect.
Depending on the scanning speed, the pulse emission timing may
substantially coincide with the phase of the interference fringes,
i.e., the moving amount of the mask between pulse emissions may
substantially coincide with the pitch of the interference fringes.
For this reason, at a given illumination point on the mask, pulse
emissions may occur in the order of positions 40C, 40F, . . . in
FIG. 5A, and at another illumination point, pulse emissions may
occur in the order of positions 40B, 40E, . . . . Therefore, the
accumulation effect cannot always be expected at every point on the
mask, and exposure amount unevenness may not necessarily.
In order to avoid this, at the scanning speed for performing pulse
emissions at positions 40C, 40F, and 40I in FIG. 5A, the
interference fringes may be laterally shifted by .delta.A upon
pulse emission at the position 40F and by .delta.B upon pulse
emission at the position 40I using, e.g., a pivot mirror. Then,
respective points in the pattern area on the mask are equally
divided in correspondence with the numbers of pulses of curves 40
(solid curve), 42 (dotted curve), and 43 (alternate long and short
dashed curve) in FIG. 5B, and are irradiated with a plurality of
pulse light components corresponding to different phases of
interference fringes. As a result exposure amount unevenness can be
remarkably reduced by the accumulation effect. In other words, the
accumulated exposure amount on the shot area on the photosensitive
substrate becomes almost even on the entire surface. Note that
illuminance unevenness in the scanning direction is reduced by
controlling the operation of the interference fringe moving member,
so that the phases in the scanning direction on the curve 40 are
respectively given by 0, 2 m.pi.+(2.pi./n), 4 m.pi.+(4.pi./n), 6
m.pi.+(6.pi./n), . . . , 2(n-1)m.pi.+2(n-1).pi./n), . . . (where n
and m are integers) for the pulse emissions at an arbitrary
radiation point on the mask.
A third scanning exposure apparatus according to the present
invention, comprises a light source for emitting pulse light having
a predetermined spatial coherency, an optical integrator for
receiving the pulse light and forming a plurality of light source
images, an optical system for focusing light components from the
plurality of light source images and forming illuminating a local
area on a mask with the focused light components, and a device for
synchronously moving the mask and a photosensitive substrate to
transfer a pattern formed on the mask onto the photosensitive
substrate. The apparatus further comprises a variable phase member
for shifting (the phase of) interference fringes formed in the
illumination area along a direction of relative scanning between
the illumination area and the mask. The contrast of the accumulated
light amount distribution on the mask or the photosensitive
substrate after radiation of the plurality of pulse light
components is set to be equal to or smaller than a predetermined
allowable value by phase modulation by a length obtained by adding
the moving amount of the interference fringes upon relative
scanning between the illumination area and the mask between pulse
emissions, and the shift amount of the interference fringes in the
relative scanning direction by the variable phase member between
pulse emissions. It is preferable that the variable phase member be
allowed to also move the interference fringes in a direction
perpendicular to the above-mentioned relative scanning
direction.
According to the third apparatus of the present invention, the
moving amount of the interference fringes in the relative scanning
direction by the variable phase member is determined in units of,
e.g., one to several pulses, in accordance with the scanning speed
of the mask (and the photosensitive substrate) and a proper
exposure amount of the photosensitive substrate. In this case, the
moving amount of the interference fringes in the relative scanning
direction by the variable phase member is determined in accordance
with the relationship between the "pitch of interference fringes
formed in the illumination area" and the "relative scanning speed
of the illumination area and the mask", so that the contrast
(residual contrast) of the accumulated light amount distribution on
the photosensitive substrate after radiation of the plurality of
pulse light components becomes equal to or smaller than the
predetermined allowable value. Therefore, even when the
relationship between the pitch of the interference fringes and the
relative scanning speed changes upon a change in proper exposure
amount of the photosensitive substrate, the residual contrast will
never become larger than the allowable value to unduly impair
exposure amount evenness.
When a one-dimensional pivot mirror (e.g., a polygonal mirror or a
galvano mirror) for moving the interference fringes in only the
scanning direction is used as the variable phase member, the pivot
control of the pivot mirror can be realized by simple reciprocal
motion if the direction (sign) of the moving amount in the relative
scanning direction of the interference scanning is taken into
consideration. Furthermore, when a pivot mirror which can
two-dimensionally oscillate is used as the variable phase member to
move the interference fringes in the illumination area additionally
in the non-scanning direction perpendicular to the relative
scanning direction, exposure amount unevenness in the non-scanning
direction can also be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing the arrangement of a
projection exposure apparatus according to the first embodiment of
the present invention;
FIG. 2 is a block diagram showing a control system of the
projection exposure apparatus shown in FIG. 1;
FIG. 3 is a view showing an example of the arrangement of a beam
shaping optical system in FIG. 1;
FIG. 4 is a perspective view showing the illuminance distribution
of an illumination area on a reticle;
FIG. 5A is a graph showing the illuminance distribution, in the
scanning direction, on the illumination area on the reticle, and
FIGS. 5B and 5C are graphs showing the illumination distribution,
in the scanning direction, on the illumination area on the reticle
as a result of moving interference fringes;
FIG. 6A is a graph showing two illuminance distributions on the
illumination area when the illumination area is irradiated with
laser beams in two directions, and FIG. 6B is a graph showing an
illumination distribution as the sum of the two illumination
distributions shown in FIG. 6A;
FIG. 7 is a perspective view showing the arrangement of a
projection exposure apparatus according to the second embodiment of
the present invention;
FIG. 8 is a view for explaining the control principle of a pivot
mirror;
FIG. 9 is an explanatory view of the principle of an interference
fringe reduction method using a pivot mirror;
FIG. 10 is a graph showing the residual contrast in the case of
FIG. 9;
FIG. 11 is a graph showing the residual contrast, in the scanning
direction, of the accumulated exposure amount obtained by scanning
exposure of the second embodiment;
FIG. 12 is a graph showing an example of a control method of the
pivot mirror in the second embodiment;
FIG. 13 is a view showing the distribution state of an equivalent
light source in correspondence with FIG. 12;
FIG. 14 is a graph showing another example of the control method of
the pivot mirror in the second embodiment; and
FIG. 15 is a flow chart showing an example of an exposure operation
in the second embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The first embodiment of the present invention will be described
below with reference to FIG. 1 to FIGS. 6A and 6B. In this
embodiment, the present invention is applied to a step-and-scan
type scanning exposure apparatus which comprises a pulse
oscillation type laser light source.
Referring to FIG. 1, a laser beam LB.sub.0 in a far ultraviolet
range (e.g., a wavelength of 248 nm) emitted from an excimer laser
light source 1 is incident on a beam shaping optical system 2,
including cylindrical lenses, via mirrors M1, M2, M3, and M4. The
sectional shape of the laser beam LB.sub.0 emitted from the excimer
laser light source 1 is an elongated rectangular shape in which the
dimension in the horizontal direction (H direction) is considerably
smaller than that in the vertical direction (V direction). The beam
shaping optical system 2 expands the dimension in the horizontal
direction of the laser beam LB.sub.0, and outputs a laser beam LB
with a sectional shape having substantially the same aspect ratio
(almost similar shape) as that of an illumination area 15 (to be
described later).
FIG. 3 shows an example of the detailed arrangement of the beam
shaping optical system 2. Referring to FIG. 3, the laser beam
LB.sub.0 from the light source 1 is transmitted through a
cylindrical lens 38 having a focal length f.sub.1 and a cylindrical
lens 39 having a focal length f.sub.2 (f.sub.2 >f.sub.1), so
that the dimension in the horizontal direction of the sectional
shape is expanded to f.sub.2 /f.sub.1 times. If the divergent angle
of the laser beam LB.sub.0 incident on the cylindrical lens 38 is
represented by .theta..sub.1, a divergent angle .theta..sub.2 of
the laser beam LB emerging from the cylindrical lens 39 is
decreased to f.sub.1 /f.sub.2 of the divergent angle .theta..sub.1.
In general, since the spatial coherence of a light beam becomes
higher as the divergent angle is smaller, the spatial coherence, in
the horizontal direction (H direction) of the laser beam LB becomes
higher than that of the laser beam LB.sub.0.
Referring back to FIG. 1, the laser beam LB emerging from the beam
shaping optical system 2 is reflected by a mirror M5 and is
incident on a beam expander (or zoom lens) 3, so that its sectional
size is expanded to a predetermined value. The collimated laser
beam LB emerging from the beam expander 3 is incident on a crystal
prism (polarization member) 4 and is split into two orthogonal
polarized light components. The two polarized light components
emerging from the crystal prism 4 are incident on a quartz glass
prism 5 for optical path correction, and their beam propagation
directions are corrected. Furthermore, the laser beams of the two
polarized light components are deflected by a pivot mirror 8 via a
first fly-eye lens 6 and a relay lens 7. The pivot mirror 8 scans,
using a driver 9, the laser beams in a predetermined angle range on
the horizontal plane by an appropriate control method.
The laser beams scanned by the pivot mirror 8 are incident on a
second fly-eye lens 11 via a relay lens 10, and form a large number
of tertiary light sources on the focal plane at the exit side of
the fly-eye lens 11. Laser beams from the large number of tertiary
light sources are focused by a focusing lens 12, are reflected by a
mirror 13, and are then incident on a condenser lens 14. The laser
beams from the large number of tertiary light sources are radiated
by the condenser lens 14 to be superposed on the rectangular
illumination area 15, having a dimension D in a short-side
direction, on a reticle R. An image of a pattern in the
illumination area 15 is imaged and projected in a rectangular
exposure area 16 on a wafer W via a projection optical system
PL.
The Z-axis is defined in a direction parallel to the optical axis
of the projection optical system PL, the X-axis in the XY plane
perpendicular to the optical axis is defined as the short-side
direction of the illumination area 15, and the Y-axis is defined as
the long-side direction of the illumination area 15. In this
embodiment, if the projection magnification of the projection
optical system is represented by .beta., the wafer W is moved at a
constant speed .beta..V in a -X direction (to be referred to as a
scanning direction SW hereinafter) in synchronism with movement of
the reticle R in the X direction (to be referred to as a scanning
direction SR hereinafter) at a constant speed V with respect to the
illumination area 15. In this manner, the image of the circuit
pattern in a pattern area PA on the reticle R is scanning-exposed
on the shot area on the wafer W.
In FIG. 1, in order to check the spatial coherence of excimer laser
light, a focusing lens L1 is arranged behind the mirror M6 to focus
light leaked from the mirror M5 at the rear-side focal point
position of the focusing lens L1, and the focused light is received
by a two-dimensional image pickup element (e.g., a CCD) 17 arranged
at the focal point position. The divergent angle of a laser beam is
measured by processing the image pickup signal from the image
pickup element 17 by an image processing system 18. Since the
divergent angle of the laser beam is inversely proportional to the
spatial coherence, the spatial coherences in the scanning direction
SR and the non-scanning direction on the illumination area 15 can
be calculated on the basis of the measured divergent angle.
FIG. 2 shows a control system of the projection exposure apparatus
shown in FIG. 1. Referring to FIG. 2, the excimer laser light
source 1 includes a laser tube 21 in which a gas mixture serving as
a medium of laser oscillation, and oscillation trigger electrodes
are sealed, a front mirror 22 having a predetermined reflectance
(less than 100%) and constituting a resonator, a rear mirror 23 of
the resonator, an aperture plate 29 for wavelength selection, a
prism 24 for wavelength selection and wavelength band narrowing, a
reflection type diffraction grating 25, and the like. Furthermore,
the excimer laser light source 1 includes an oscillation controller
26 for applying a high voltage to the electrodes in the laser tube
21 to perform oscillation, a wavelength adjustment drive unit 27
for adjusting the inclination angle of the diffraction grating 25
so as to always make constant the absolute wavelength of a laser
beam to be oscillated, a drive unit 28 for adjusting the
inclination of the rear mirror 23, and the like.
Some light components of a laser beam emerging from the front
mirror 22 are guided to a wavelength detector (beam splitter or the
like) 31 via a beam splitter 30. The wavelength detector 31 detects
the wavelength of the laser beam, and supplies the detected
wavelength to the wavelength adjustment drive unit 27. The
wavelength adjustment drive unit 27 changes the inclination angle
of the diffraction grating 25 in accordance with the wavelength
detected by the wavelength detector 31, so that the difference
between the detected wavelength and a predetermined absolute
wavelength becomes equal to or smaller than a prescribed value. A
signal corresponding to the beam divergent angle detected by
processing the image pickup signal from the image pickup element 17
by the image processing system 18 (more specifically, a signal
corresponding to the size of the beam spot formed on the
light-receiving surface of the image pickup element 17) is fed back
to the drive unit 28 of the rear mirror 23 in the excimer laser
light source 1, and is also supplied to a main controller 32 for
controlling the operation of the entire apparatus. When the
actually measured value of the beam divergent angle is different
from a predetermined value beyond an allowable range, the drive
unit 28 changes the inclination angle of the rear mirror 23.
A reticle stage RST performs alignment and scanning of the reticle
R, and a wafer stage WST performs alignment and scanning of the
wafer W. The reticle stage RST scans the reticle R to sequentially
change the radiation range on the reticle R on which a 1-chip
pattern is formed. The wafer stage WST has both a function of
moving the wafer W in the X and Y directions by the step-and-repeat
method, and a function of scanning the wafer W in synchronism with
the scanning of the reticle R in correspondence with the radiation
range on the reticle R, so that a pattern image on the reticle R is
exposed on each of a plurality of shot areas on the wafer W.
The main controller 32 controls oscillation of the excimer laser
light source 1 via the oscillation controller 26, and controls the
operations of the wafer stage WST and the reticle stage RST via a
wafer stage controller 34 and a reticle stage controller 35,
respectively. Furthermore, the main controller 32 controls the
amplitude, cycle, and the like of the pivotal motion of the pivot
mirror 8 via the driver 9. The main controller 32 is connected to a
keyboard 36 as an input device, a coordinate input device
(so-called mouse) 37, a display (CRT display, meter, or the like)
33 as an output device, and the like. The keyboard 36 and the
coordinate input device 37 are used for designating, in advance,
the number of shots of pulse light for exposing a single shot area
on the wafer, and for setting various sequences and parameters.
The main controller 32 receives information of the divergent angle
of the laser beam from the excimer laser light source 1 which is
executing preliminary oscillation from the image processing system,
and determines an oscillation frequency which is optimized to
minimize the contrast of interference fringes without lowering the
throughput, and the number of pulses of a laser beam to be radiated
onto a single shot area on the wafer W. Thereafter, the main
controller 32 issues a command to the oscillation controller 26. At
the same time, the main controller 32 determines the pivot cycle,
amplitude, and phase of the pivot mirror 8, and issues a command to
the driver 9. Also, the main controller 32 determines optimal
scanning speeds, and issues commands to the reticle and wafer stage
controllers 35 and 34.
Next, the arrangement for reducing light amount unevenness on the
reticle R and wafer W in this embodiment will be explained. In this
embodiment, the spatial coherence of the laser beam LB.sub.0
emitted from the excimer laser light source 1 in FIG. 1 is high in
the horizontal direction (H direction). Thus, an illumination
optical system is constituted, so that a direction with a higher
spatial coherence of the laser beam LB.sub.0 coincides with the
short-side direction of the illumination area 15, i.e., the
scanning direction SR. With this arrangement, interference fringes
(speckle pattern) formed in the illumination area 15 on the reticle
R have a high contrast in the scanning direction SR, and have a low
contrast in a non-scanning direction (Y direction) perpendicular to
the scanning direction SR.
The interference pattern formed on the reticle R and the wafer W in
FIG. 1 includes periodic components corresponding to the
arrangement of lens elements of the fly-eye lenses 6 and 11, and
has a higher contrast in the scanning direction than that in the
non-scanning direction. For this reason, in this embodiment, in
order to reduce the contrast of the interference pattern, the laser
beam LB is split into laser beams of two polarized light components
which define a predetermined angle therebetween, and these laser
beams are radiated onto the reticle R. An illuminance distribution
(relative value) I(X), in the scanning direction (X direction), on
the illumination area 15 with the first polarized light component
of the two polarized light components periodically changes at a
predetermined pitch, as indicated by a curve 40 in FIG. 6A. An
illuminance distribution I(X) based on the second polarized light
component is indicated by a curve 44, and the curve 44 is shifted
by half a pitch from the curve 40 in the X direction. Thus, the
entire illuminance distribution I(X) is expressed by a curve 45 in
FIG. 6B, and the variation width of the illuminance distribution is
greatly reduced.
FIG. 4 shows the illuminance distribution of the illumination area
15 on the reticle R. On the reticle R, the illumination area 15
having the dimension D in the scanning direction SR (X direction)
is formed. The illuminance distribution I(X), in the X direction,
of the illumination area 15 changes at a predetermined pitch and
with a relatively large amplitude, as indicated by the curve 40,
and the illuminance distribution I(Y), in the Y direction, of the
illumination area 15 is almost flat, as indicated by a curve 41.
Therefore, light amount unevenness in the non-scanning direction (Y
direction) becomes small. In this embodiment, light amount
unevenness in the X direction is reduced by scanning the reticle R
with respect to the illumination area 15 and by scanning a laser
beam by the pivot mirror 8.
FIG. 5A shows the curve 40 corresponding to the illuminance
distribution I(X), in the scanning direction (X direction) per
pulse, on the illumination area 15, and an area from an origin 0 to
an X-coordinate position D corresponds to the dimension, in the X
direction of the illumination area 15 in FIG. 4. Assume that upon
scanning of the reticle R in the X direction with respect to the
illumination area 15, respective radiation points on the reticle R
move along the X-axis in FIG. 5A (the same applies to FIG. 5B).
In this embodiment, if the pitch of the curve 40 is represented by
PX and the number of pulses required for exposure of one shot area,
which is calculated based on the energy density per pulse and the
resist sensitivity, is represented by n, when a scanning speed
which yields a curve having peaks at positions 0, PX/n, 2PX/n, . .
. , (n-1)PX/n for n pulse emissions coincides with a predetermined
speed (a value V=(D/n)f obtained by dividing the dimension D of the
illumination area 15 with the required number n of pulses, and
multiplying the product with an oscillation frequency f of the
light source 1), accumulated light amount unevenness on the reticle
R and the wafer W is most efficiently reduced whether or not the
pivot mirror 8 is scanned. Note that the above-mentioned scanning
speed need not yield a single curve having peaks in the order of 0,
PX/n, 2PX/n, . . . , (n-1)PX/n, but need only yield all curves to
provide peaks at these positions in n pulse emissions. Also, in
some cases, a curve having peaks at positions obtained by equally
dividing the pitch PX by n/2, n/3, . . . need only be obtained.
For example, when the required number n of pulses is 3, the reticle
R moves by D/3 in the X direction for each pulse. Then, as shown in
FIG. 5A, at a given radiation point (X=0) on the reticle R, pulse
light is radiated in the order of positions 40A, 40E, 40I, . . . at
intervals D/3. Since the accumulated light amount distribution in
the X direction is obtained by superposing pulse light components
having the illuminance distributions indicated by the curves 40,
42, and 43 in FIG. 5B, accumulated light amount unevenness becomes
very small. The moving distance per pulse of the reticle R is set
to be a unit fraction of the dimension D in the scanning direction
SR.
However, since the scanning speeds of the reticle R and the wafer W
are determined on the basis of a proper exposure amount on the
wafer W, and the like, as will be described later, the
above-mentioned condition may not always be satisfied. In such a
case, the laser beam can be controlled using the pivot mirror 8 to
obtain an illuminance distribution having peaks at positions 0,
PX/n, 2PX/n, . . . , (n-1)PX/n.
Assume for example, that the required number n of pulses is 4, and
the reticle R is moved by D/4 in the X direction for each pulse.
Then, as shown in FIG. 5A, at a given radiation point (X=0) on the
reticle R, pulse light is radiated in the order of positions 40A,
40D, 40G, 40K, . . . at intervals D/4, and at another given point,
e.g., a point separated by D/6 from the position of X=0, pulse
light is radiated in the order of positions 40C, 40F, 40I, and 40L.
As a result, the accumulated light amount distribution in the X
direction is expressed by superposing the curves 40, and light
amount unevenness is not reduced at all. Thus, in this case, the
pivot mirror 8 is scanned. For example, when the phase of the
illuminance distribution (interference fringes) is changed by
scanning the pivot mirror 8 by PX/4 at the position 40F, by PX/2 at
the position 40I, and by 3PX/4 at the position 40L, the accumulated
light amount distribution is expressed by superposing waves with
four different phases, as shown in FIG. 5C, and light amount
unevenness becomes very small. Curves 46, 47, and 48 in FIG. 5C are
obtained by respectively changing the phase of the curve 40 by
PX/4, PX/2, and 3PX/4 using the pivot mirror 8.
The scanning speeds of the reticle R and the wafer W will be
explained below. The scanning speed of the wafer W is determined
based on a proper exposure amount (determined in accordance with
the sensitivity characteristics of a resist coated on the wafer W)
to be given to the wafer W, and the energy amount per pulse. In the
case of the excimer laser light source 1, the energy amount
discharged per pulse varies within a range of, e.g., about .+-.5%.
For this reason, the intensity (energy amount) of the laser beam to
be incident on the reticle R is attenuated, the number of pulses
required for scanning exposure of one shot area is increased, and
the energy amount per pulse is determined to decrease the variation
in light amount on the wafer W by the accumulation effect.
If the proper exposure amount of the wafer W is represented by E
and the energy amount per pulse (average energy amount) is
represented by E.sub.p, a minimum number of exposure pulses
required at a predetermined point on the wafer W is given by
E/E.sub.p. Since the length (the dimension, in the scanning
direction of the illumination area 15), in the scanning direction,
of a range which is simultaneously irradiated with light on the
reticle R is D, the moving amount per pulse of the reticle R is
given by (E.sub.p /E)D. Therefore, when the oscillation frequency
of the excimer laser light source 1 is f [Hz], a scanning speed V
of the reticle R is set to be a value given by the following
equation:
In this embodiment, the interference pattern in the illumination
area 15 is not moved in the non-scanning direction (Y direction in
FIG. 4). However, in order to further reduce light amount
unevenness in the non-scanning direction, it is desirable that, for
example, the pivot mirror 8 in FIG. 1 be arranged to allow
two-dimensional pivotal motion so as to scan the interference
pattern also in the non-scanning direction. Alternatively, two sets
of pivot mirrors may be arranged in the illumination optical system
to independently shift the interference pattern in the scanning and
non-scanning directions. Also, in order to move the interference
pattern in both the scanning direction SR (X direction) and the
non-scanning direction (Y direction) in FIG. 4, the interference
pattern may be shifted in a direction (e.g., a 45.degree.
direction) crossing the X and Y directions.
As the method of causing the direction with a higher spatial
coherence to coincide with the scanning direction, the following
techniques are also available.
1 If the exposure apparatus main body is arranged to be able to
scan the reticle and wafer in both the X and Y directions, even
after the apparatus main body and the laser light source are
connected, one of the X and Y directions corresponding to a higher
spatial coherence may be selected as the scanning direction. The
shape of the illumination area may be set by, e.g., a reticle blind
(field stop), to assure that the selected scanning direction
coincides with the short-side direction of the illumination area on
the reticle.
2 In order to cause a direction corresponding to a higher spatial
coherence of laser light from the light source to coincide with the
scanning direction, the direction, corresponding to a higher
spatial coherence of a laser beam emitted from the light source and
incident on the illumination optical system of the exposure
apparatus can be adjusted using, e.g., a plurality of mirrors. In
this case, the fly-eye lenses and the like must often be adjusted.
In general, it is desirable to assemble the apparatus in
consideration of the direction corresponding to a higher spatial
coherence.
In this embodiment, the interference pattern is moved for each
pulse by the pivot mirror 8. Alternatively, the interference
pattern may be moved for every several pulses.
The second embodiment of the present invention will be described
below with reference to FIG. 7. FIG. 7 shows the arrangement of a
scanning projection exposure apparatus according to the embodiment
comprising a pulse oscillation type laser light source. The same
reference numerals in FIG. 7 denote parts having the same functions
and effects as those in FIG. 1.
Referring to FIG. 7, a laser beam LB.sub.0 in a far (or deep)
ultraviolet range (e.g., a wavelength of 248 nm) emitted from an
excimer laser light source 1 is incident on a beam shaping optical
system 2 including cylindrical lenses. In general, the sectional
shape of the laser beam LB.sub.0 emitted from the excimer laser
light source 1 is an elongated rectangular shape in which the
dimension in the horizontal direction (H direction) is considerably
smaller than that in the vertical direction (V direction). The beam
shaping optical system 2 shapes the laser beam LB.sub.0 into a beam
which has a square section with an aspect ratio of 1 : 1, and
outputs the shaped beam.
The laser beam emerging from the beam shaping optical system 2 is
deflected by mirrors M1 and M2 and is incident on a beam expander
3, so that its sectional dimension is expanded to a predetermined
value. A collimated laser beam LB emerging from the beam expander 3
is reflected by a mirror M3, and thereafter, its optical path is
deflected by a pivot mirror (phase modulator for interference
fringes) 54. The pivot mirror 54 is supported to be allowed to
independently oscillate in two directions to have two orthogonal
axes 54a and 54b as rotation axes, and two motors (not shown) for
oscillating the pivot mirrors 54 about the two pivot axes 54a and
54b suitably arranged.
The laser beam reflected by the pivot mirror 54 is incident on an
optical integrator (fly-eye lens) 55 via a field lens 61 and an
input lens 62 (not shown in FIG. 7), which are shown in FIG. 8. The
fly-eye lens 55 is constituted by aligning small lens elements each
with a rectangular section in the vertical and horizontal
directions to be in tight contact with each other, and a large
number of light source images (secondary light sources) are formed
at the rear-side (reticle-side) focal plane of the fly-eye lens 55.
Some of laser beams diverging from the large number of light source
images are reflected by a beam splitter 56, and are then incident
on a photoelectric detector (integrator sensor) 57 via a focusing
optical system (not shown).
The laser beams transmitted through the beam splitter 56 are
focused by a first relay lens 58 on a reticle blind (field stop) 59
arranged in a plane substantially conjugate with the pattern
formation surface of a reticle R. Therefore, the shape of an
illumination area 15 on the reticle R is determined by the aperture
shape of the reticle blind 59. In this embodiment, the shape of the
illumination area 15 is a rectangular shape having a dimension D in
its short-side direction (scanning direction SR). The laser beams
passing through the aperture of the reticle blind 59 are incident
on only a portion (illumination area 15) in a pattern area on the
reticle R via a second relay lens 60, a mirror 13, and a condenser
lens 14. More specifically, the laser beams from the large number
of light source images formed by the fly-eye lens 55 illuminate the
illumination area 15, with the dimension D in the short-side
direction on the reticle R, to be superposed on each other via the
condenser lens 14. An image of a pattern in the illumination area
15 is imaged and projected in a rectangular exposure area 16 on a
wafer W via a projection optical system PL. Note that interference
fringes are formed in the illumination area 15 in accordance with
the arrangement of light source images in the rear-side focal plane
of the fly-eye lens 55, and the following description will be given
under an assumption that interference fringes are formed along the
X and Y directions.
The control method of the pivot mirror 54 will be described in
detail below. FIG. 8 is a diagram for explaining the basic
principle of control. FIG. 8 shows only the principal members in
FIG. 7, and the X-axis, i.e., the axis of scanning directions SR
and SW, is defined to be parallel to the plane of the drawing of
FIG. 8. A case will be examined below wherein the pivot mirror 54
is driven to one-dimensionally scan the interference fringes in the
scanning direction only.
Referring to FIG. 8, a laser beam LB emerging from the beam
expander 3 consisting of lens elements 3a and 3b is reflected by
the pivot mirror 54 which is pivotal at high speed about the axis
54a perpendicular to the plane of the drawing of FIG. 8. The beam
is thereafter focused on a plane 63 by the field lens 61, thus
forming an equivalent light source LS on the plane 63. A laser beam
from the equivalent light source LS is incident on the fly-eye lens
55 via the input lens 62. Furthermore, laser beams from the large
number of light source images formed on the rear-side focal plane
of the fly-eye lens 55 illuminate the illumination area 15 on the
reticle R to be superposed on each other via the condenser lens
14.
The reticle R is held on a reticle stage RST, and the reticle stage
RST scans the reticle R with respect to the illumination area 15 at
a predetermined speed in the direction SR or in an opposite
direction. On the other hand, the wafer W is placed on a wafer
stage WST, and the wafer stage WST sequentially sets an end portion
of each shot area on the wafer W within the image field of the
projection optical system PL by a stepping operation. More
specifically, the wafer stage WST sets each shot area at a
predetermined approach (acceleration) start position in a
rectangular coordinate system XY, and thereafter scans the wafer W
at a predetermined speed in the direction SW or in an opposite
direction in synchronism with the scanning operation of the reticle
stage RST. The projection optical system PL is constituted by a
front-group lens system 64, an aperture stop 65, and a rear-group
lens system 66. Note that the aperture stop 65 is arranged on the
pupil plane of the projection optical system, and defines the
numerical aperture (NA) of the optical system.
In this embodiment, since the pivot mirror 54 is pivoted, i.e.,
oscillated about the shaft 54a, the equivalent light source LS on
the plane 63 is caused to have a predetermined size as a time
average, thus achieving incoherency on the illumination area 15.
Since the excimer laser light source 1 has a very high directivity,
the intensity distribution of the equivalent light source LS at a
given timing can be processed as a .delta. function having a sharp
peak at only a given point. On the other hand, when the pivot
mirror 54 stands still, interference fringes at a pitch P are
formed on the pattern formation surface of the reticle R by
interference among the light of the large number of lens elements
of the fly-eye lens 55. In this case, if .lambda. represents the
wavelength (exposure wavelength) of the laser beam LB, P.sub.s
represents the lens element interval of the fly-eye lens 55 in the
scanning direction, f.sub.c represents the focal length of the
first relay lens 58, and .alpha. represents the magnification of an
optical system (60, 14) arranged between the reticle blind 59 and
the reticle R, the pitch P of the interference fringes is given
by:
Next, assume that the pivot mirror 54 is oscillated under a
condition that the pivoting angle of the pivot mirror 54 between
pulse emissions is .theta..sub.0 and the number of exposure pulses
on one shot area is N pulses not in the scanning exposure method
but in a conventional still exposure method (stepper method), as
shown in FIG. 9. Note that the pivoting angle .theta..sub.0
represents the size of the equivalent light source LS between pulse
emissions, and when the pivoting angle of the pivot mirror 54 is
small, the pivoting angle .theta..sub.0 and the actual pivoting
angle have a correlation therebetween. After radiation of N shots
of pulse light, the contrast of the accumulated light amount
distribution on the wafer W is expressed by the square of a Fourier
transform of the distribution of the equivalent light source shown
in FIG. 9. FIG. 10 shows the contrast of the accumulated light
amount distribution on the wafer W corresponding to FIG. 9. In FIG.
10, the contrast of the interference fringes obtained when the
reticle R and the wafer W stand still and only one light pulse is
emitted is set to be "1".
In this embodiment, as the exposure amount control method in the
scanning exposure apparatus, the following method will be examined.
Also, light amount unevenness on the reticle R will be examined
below. The dimension, in the short-side direction (scanning
direction SR), of the illumination area 15 on the reticle R in FIG.
7 is D, and the dimension D is measured by a technique of, e.g.,
multiplying a length obtained, in advance, by scanning a
photoelectric conversion element on the wafer stage on the image
plane with the reciprocal (1/.beta.) of a projection magnification
D of the projection optical system PL.
Assume that the number of exposure pulses for an arbitrary point in
the illumination area 15 on the reticle R is represented by N. The
number N of exposure pulses is calculated based on the proper
exposure amount of the wafer W and the energy amount per pulse of
the laser beam. During scanning exposure, the reticle stage RST
moves by D/N along the scanning direction SR until the next pulse
is radiated. If the variation (standard deviation .sigma. or
3.sigma.) of the energy amount e per pulse of the laser beam is
represented by .delta.e, and the average value of the pulse energy
e is represented by E, the variation in pulse energy is normalized
to .delta.e/E. If the reproducibility (exposure amount control
accuracy) of the proper exposure amount is represented by A, the
number N of exposure pulses has the following lower limit N.sub.min
:
If the pulse energy e of the laser beam is large and the number N
of exposure pulses becomes equal to or smaller than N.sub.min, the
condition given by relation (2) is satisfied by inserting a light
attenuation member such as an ND filter in the optical path of the
laser beam LB (or LB.sub.0). When the number N of exposure pulses
is determined in this manner, the reticle R moves by D/N in the
scanning direction SR for each pulse emission. This moving amount
corresponds to D/(NP) (unit: the number of cycles of interference
fringes) if the pitch P (determined by equation (1)) of the
interference fringes is used as a unit, and the moving amount is
equivalent to a state wherein the interference fringes are shifted
by D/(NP) (cycles) on the reticle R in the scanning direction SR
during scanning exposure, even when the pivot mirror 54 stands
still. In the following description, the pivoting angle of the
pivot mirror 54 is expressed in units of the moving amount of
interference fringes (the number of cycles of moving interference
fringes) on the reticle R. Also, in the following description,
assume that the pivoting angle of the mirror and the moving amount
of interference fringes on the reticle R have a correlation
therebetween.
FIG. 11 shows the contrast (residual contrast) of interference
fringes after accumulation exposure is performed using Nm shots of
pulse light in this embodiment, in correspondence with FIG. 10. In
FIG. 11, the pivoting angle pitch (pivoting angle between
emissions) of the pivot mirror 54 in units of cycles of
interference fringes is plotted along the abscissa, and the
residual contrast is plotted along the ordinate. The number N.sub.m
of pulses represents the number of exposure pulses within a half
cycle of the pivot cycle of the pivot mirror 54 (the details will
be described later). In this case, since each position where the
value plotted along the abscissa is an integer corresponds to a
case wherein interference fringes are shifted by integer cycles and
are superposed on each other, the residual contrast assumes 1 as a
maximum value, and small peaks whose positions are determined by an
integer N.sub.m continuously appear around the maximum peak. In
this embodiment, the movement of interference fringes on the
reticle R upon driving of the reticle stage during scanning
exposure is also equivalently processed as a result of pivoting the
pivot mirror 54. More specifically, with the number N of exposure
pulses, it is assumed that a pivoting angle offset D/(NP) is added
to the pivoting angle in FIG. 11 even when the pivot mirror 54
stands still in practice.
As can be seen from FIG. 11, when the pivoting angle offset D/(NP)
assumes an integer or a numerical value very close to an integer,
if the pivot mirror 54 is kept inactive, interference fringes with
a high contrast remain after the accumulation exposure using
N.sub.m shots of pulse light. To prevent this, in this embodiment,
the residual contrast is controlled to fall within a safe area
S.sub.+ A or a safe area S.sub.- A in FIG. 11 by the pivot control
of the pivot mirror 54. More specifically, each of the safe areas
S.sub.+ A and S.sub.- A corresponds to an area where the residual
contrast value becomes equal to or smaller than a predetermined
value, and in FIG. 11, areas excluding five peaks having the
maximum peak at the central position are respectively the safe
areas S.sub.+ A and S.sub.- A. Areas other than the safe areas
S.sub.+ A and S.sub.- A on the abscissa in FIG. 11 are danger areas
DA where the residual contrast is high, and light amount unevenness
may exceed an allowable value.
In practice, the setting method of the safe areas S.sub.+ A and
S.sub.- A is determined on the basis of the characteristics of a
laser light source to be used, the photosensitive characteristics
of a photoresist to be coated on the wafer W, the allowable value
of light amount unevenness, and the like. Normally, these areas can
be set assuming the worst conditions. Referring to FIG. 11, the
control method of the pivot mirror 54 in FIG. 8 is classified into
the following two cases. [When pivot mirror 54 is driven (condition
A)]
When the pivoting angle offset D/(NP) falls within a danger area
DA, the pivot mirror 54 is controlled, so that {D/(NP).+-..delta.}
(where .delta. is the pivoting angle (pivoting pitch) between pulse
emissions of the pivot mirror 54) falls within, e.g., a safe area
S.sub.+ A. Note that the signs .+-. of the pivoting angle 6 have
the following meanings. The sign + indicates that the scanning
direction of the reticle R is the same as the moving direction of
interference fringes by the pivot mirror 54, and the sign -
indicates that the two directions are opposite to each other.
In the example shown in FIG. 11, if the interval between the
pivoting angle offset D/(NP) and the safe area S.sub.- A is
represented by .delta..sub.- (negative value) and the interval
between the pivoting angle offset D/(NP) and the safe area S.sub.+
A is represented by .delta..sub.+ (positive value),
.vertline..delta..sub.+.vertline.>.vertline..delta..sub.-.vertline.
is satisfied. Therefore, if a larger one of values a and b is
expressed by max(a, b), the pivoting angle .delta. of the pivot
mirror 54 is given by the following equation:
[When pivot mirror 54 can be kept inactive (condition B)]
When the pivoting angle offset D/NP falls within the safe area
S.sub.+ A or S.sub.- A, the pivot mirror 54 need not be
pivoted.
Four conditions upon transfer of an image of a pattern on the
reticle R onto the wafer W by the scanning exposure method in this
embodiment will be explained below.
First condition
An arbitrary point in the illumination area 15 on the reticle R
must be illuminated with light from the equivalent light source LS
(see FIG. 8) having the same size.
Second condition
If the pivot cycle of the full stroke of the pivot mirror 54 is
represented by T.sub.M, the number of exposure pulses in a half
cycle for an arbitrary point in the illumination area 15 must be
determined, so that the arbitrary point is illuminated for a time
corresponding to an integer multiple of the half cycle (T.sub.M
/2). With this condition, even when a control method for
reciprocally moving the pivot mirror 54 is adopted, interference
fringes can be shifted for a time corresponding to an integer
multiple of the half cycle (T.sub.M /2) upon illumination of all
the points on the reticle R.
Third condition
In order to lower the residual contrast of interference fringes
after scanning exposure on the reticle R (or the wafer W), it is
desirable that the number of exposure pulses in the half cycle of
the pivot mirror 54 be as large as possible to enhance the
integration effect.
Fourth condition
When a piezoelectric element or the like is used as a driver for
the pivot mirror 54, the pivoting angle .delta. of the mirror 54
between pulse emissions has a predetermined resolution. For
example, in the case shown in FIG. 11, since a minimum value of the
pivoting angle between pulse emissions is 3/N.sub.m, the maximum
number of exposure pulses in the half cycle must be determined so
that the minimum value becomes equal to or larger than the
resolution of the pivoting angle of the pivot mirror 54.
In consideration of the above-mentioned conditions, the control
sequence of the pivot mirror 54 in this embodiment is determined.
FIGS. 12 to 14 show an example of the control sequence. At a point
A in a shot area on the wafer W, when viewed from the pivot mirror
54 in FIG. 7, exposure starts at a point A.sub.s, and after the
pivoting angle changes by one cycle in a triangular wave pattern
having an amplitude .theta..sub.0, the exposure ends at a point
A.sub.f. At a point B slightly shifted from the point A in the
scanning direction, when viewed from the pivot mirror 54, exposure
starts at a point B.sub.s, and ends at a point B.sub.f after the
pivoting angle changes by one cycle. More specifically, both the
points A and B are exposed while the pivoting angle of the pivot
mirror 54 changes by one cycle with the full stroke
.theta..sub.0.
FIG. 13 shows the movement of the equivalent light source LS in
FIG. 8 in correspondence with FIG. 12. As shown in FIG. 13, the
equivalent light source LS moves to cyclically oscillate with the
full stroke .theta..sub.0 along the X-axis. In association with
this movement, for the point A on the wafer W, the equivalent light
source LS moves by one cycle from the point A.sub.s to the point
A.sub.f, and for the point B on the wafer W, the equivalent light
source LS moves by one cycle from the point B.sub.s to the point
B.sub.f.
An example of the scanning exposure operation in this embodiment
will be described below with reference to FIG. 15. Of minimum
numbers of exposure pulses which satisfy relation (2), even numbers
are represented by N.sub.min. Furthermore, from the above-mentioned
first condition, the minimum number N.sub.m,min of exposure pulses
in the half cycle (T.sub.M /2) of the pivotal motion of the pivot
mirror 54 is given by:
From the above-mentioned fourth condition, the maximum number of
exposure pulses in the half cycle of the pivotal motion of the
pivot mirror 54 is represented by N.sub.m,max. In step 101 in FIG.
15, the number N of exposure pulses per point on the wafer W is
determined to satisfy relation (2). In step 102, the number N.sub.m
of exposure pulses in the half cycle of the pivotal motion of the
pivot mirror 54 is determined. The number N.sub.m of exposure
pulses satisfies the following condition:
Therefore, if N.sub.min /2.ltoreq.N.sub.m.ltoreq.N.sub.m,max is
satisfied, the number N.sub.m of exposure pulses is given by the
following equation (step 103) (int(a) represents the integral part
of a real number a):
On the other hand, if 2N.sub.m,max.ltoreq.N, the number N.sub.m of
exposure pulses is given by the following equation (step 104):
In this manner, the first to fourth conditions are satisfied. After
execution of step 103 or 104, the flow advances to step 105 to
check if drive control of the pivot mirror 54 is to be executed. In
this case, the safe areas of the pivoting angle of the pivot mirror
54 are assumed to be the safe areas S.sub.+ A and S.sub.- A in FIG.
11. If CINT(a) represents an integer closest to a real number a,
i.e., an integer obtained by rounding the first digit below the
decimal point of the real number a, the pivot control of the pivot
mirror 54 is performed when .vertline.D/(NP)-CINT(D/(DP)).vertline.
as a shift amount from a position corresponding to the maximum
residual contrast is smaller than 3/N.sub.m as an allowable value.
More specifically, when the following inequality is satisfied, the
flow advances to step 107:
On the other hand, if inequality (8) is not satisfied, the flow
advances to step 106, and scanning exposure is performed while the
pivot mirror 54 stands still. In this case, the above-mentioned
condition B is used.
In step 107, the pivoting angle (pivoting angle pitch) .delta. per
pulse of the pivot mirror 54 is calculated from the following
equation using the above-mentioned condition A:
In step 108, the full stroke .theta..sub.0 of the pivoting angle of
the pivot mirror 54 is calculated from the following equation using
the pivoting angle pitch .delta. and the number N.sub.m of exposure
pulses of the half cycle:
As described above, the control variables for the pivot mirror 54
are calculated. Thereafter, pulse emission is triggered in step
109, and the pivoting angle of the pivot mirror 54 is controlled on
the basis of the calculated control variables in step 111.
Thereafter, steps 109 to 111 are repetitively executed, and if it
is determined in step 110 that scanning exposure on the entire
surface of one shot area on the wafer W is finished, the exposure
operation ends. Also, scanning exposure is performed on other shot
areas on the wafer W in the same sequence as described above.
Referring back to FIG. 7, a method of moving interference fringes
in the non-scanning direction (Y direction) on the reticle R will
be described below. As described above, in the scanning direction,
the pivot mirror 54 need not always be driven, depending on
conditions such as the proper exposure amount of the wafer W, and
the like. However, in the non-scanning direction, since the reticle
R stands still, it is desirable to shift the interference fringes
in the non-scanning direction via the pivot mirror 54 or another
scanning member.
In the non-scanning direction, the pivot mirror 54 can be pivoted
in the same manner as the conventional still exposure method
(stepper method). In the non-scanning direction, although the
residual contrast shown in FIG. 11 remains on the reticle R, there
is no pivoting angle offset of the pivot mirror 54 determined by a
proper exposure amount, unlike in the scanning direction.
Therefore, the pitch and full stroke of the pivoting angle of the
pivot mirror 54 will not change depending on the proper exposure
amount. Also, in place of the concept of target safe areas in the
scanning direction, a target point can be considered. More
specifically, since the pivoting angle offset is 0, a pivoting
angle which has a point T where the residual contrast becomes 0 as
a target point in FIG. 11 is assumed. Then, the pivoting angle of
the pivot mirror 54 is controlled, so that the pivoting angle
(pivoting angle pitch) between pulse emissions of the pivot mirror
54 corresponds to 3/N.sub.m and the full stroke .theta..sub.0
corresponds to three pitches of the interference fringes on the
reticle R. In addition, the number N.sub.m of exposure pulses in
the half cycle of the pivotal motion of the pivot mirror 54 need
only satisfy the following condition:
Note that the target point like the point T in FIG. 11 is
determined depending on the control accuracy of the pivot mirror 54
and the spatial coherency of a laser beam in the non-scanning
direction.
As described above, in this embodiment, the pitch and stroke of the
pivoting angle of the pivot mirror 54 and the number of exposure
pulses in the half cycle are changed in accordance with the number
N of pulses for one point in the shot area on the wafer W in the
scanning direction, but the pitch and stroke of the pivoting angle
are left unchanged in the non-scanning direction. As described
above, the pivot mirror 54 is controlled by different methods in
the scanning and non-scanning directions. In this embodiment, a
case has been examined wherein the pivot mirror 54 is reciprocally
moved during scanning exposure for one shot. Alternatively, a
control method for pivoting the pivot mirror 4 in only one
direction is also available.
FIG. 14 shows a case wherein the pivot mirror 54 is pivoted in one
direction. In FIG. 13, the pivot mirror 54 is pivoted at the same
angular velocities in both the forward and backward paths by
contrast, in FIG. 14, an operation for pivoting the pivot mirror
in, e.g., the forward path in the same manner as in FIG. 13, and
returning the mirror to an initial position at high speed in the
backward path is repeated, so that interference fringes are always
shifted in only a predetermined direction. If the pivot mirror 54
is constituted, by e.g., a polygonal mirror, the operation shown in
FIG. 14 can be realized by setting a constant rotation direction of
the polygonal mirror.
When the pivot mirror 54 is pivoted in one direction in this
manner, if the pivoting angle offset is D/(NP) in FIG. 11, the
target area is set to be the safe area S.sub.- A. More
specifically, in this case, the pivoting angle pitch .delta. is
given by the following equation:
The pivoting direction of the pivot mirror 54 is a direction in
which the interference fringes move in a direction opposite to the
scanning direction SR of the reticle stage. In this case, the
number N.sub.m of pulses in the half cycle of the pivotal motion of
the pivot mirror 54 is classified as follows.
When N.sub.min.ltoreq.N.ltoreq.N.sub.m,max, N.sub.m is given
by:
On the other hand, when N.sub.m,max <N, N.sub.m is given by:
Thus, the full stroke .theta..sub.0 of the pivoting angle of the
pivot mirror 54 is given by:
In this method, selection of the pivoting angle .delta. in step 107
in FIG. 15 is not required, and the maximum full stroke
.theta..sub.0 required for the pivot mirror 54 can be half that of
the method for reciprocally moving the mirror.
In this embodiment, the direction corresponding to a higher
coherency can be caused to coincide with the scanning direction SR
as in the first embodiment. Light amount unevenness in the scanning
direction can be decreased, and the contrast of exposure amount
unevenness in the non-scanning direction can also be decreased.
The present invention is not limited to the above embodiments. For
example, when continuous light such as harmonics of a YAG laser, an
emission line (i-line or the like) of a mercury lamp, or the like
is used, suitable arrangements can be achieved without departing
from the scope of the present invention.
* * * * *